In the past, a control system of an internal combustion engine which is provided with an air-fuel ratio sensor at an upstream side, in a direction of exhaust flow, of an exhaust purification catalyst, and is provided with an oxygen sensor at a downstream side thereof, in the direction of exhaust flow has been known (for example, PTL 1). In such a control system, for example, feedback control is performed based on the output of the upstream side air-fuel ratio sensor so that the output of this air-fuel ratio sensor becomes a target value corresponding to the target air-fuel ratio. In addition, the target value of the upstream side air-fuel ratio sensor is adjusted based on the output of the downstream side oxygen sensor. Note that, in the following explanation, the upstream side in the direction of exhaust flow will sometimes be simply referred to as the “upstream side”, and the downstream side in the direction of exhaust flow will sometimes be simply referred to as the “downstream side”.
For example, in the control system described in PTL 1, when the output voltage of the downstream side oxygen sensor is a high side threshold value or more and thus the exhaust purification catalyst is in an oxygen deficient state, the target air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst is set to an air-fuel ratio which is leaner than the stoichiometric air-fuel ratio (below, also referred to as the “lean air-fuel ratio”). Conversely, when the output voltage of the downstream side oxygen sensor is the low side threshold value or less and thus the exhaust purification catalyst is in an oxygen excess state, the target air-fuel ratio is set to an air-fuel ratio which is richer than the stoichiometric air-fuel ratio (below, also referred to as the “rich air-fuel ratio”). According to PTL 1, due to this, when the catalyst is in the oxygen deficient state or oxygen excess state, it is considered possible to quickly return the state of the exhaust purification catalyst to an intermediate state between the two states (that is, state where the exhaust purification catalyst stores a suitable amount of oxygen).
In addition, in the above control system, when the output voltage of the downstream side oxygen sensor is between the high side threshold value and low side threshold value, when the output voltage of the oxygen sensor is increasing as a general trend, the target air-fuel ratio is set to a lean air-fuel ratio. Conversely, when the output voltage of the oxygen sensor is decreasing as a general trend, the target air-fuel ratio is set to a rich air-fuel ratio. According to PTL 1, due to this, it is considered possible to prevent in advance the exhaust purification catalyst from becoming in an oxygen deficient state or in an oxygen excess state.